Javascript functions interview questions, Closures & ES6 Part 2

javascript functions interview questions including closures, scope, IIFE, prototypes, and currying. Essential for frontend developers preparing for technical interviews in 2025. Javascript functions interview questions

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🚀 Javascript functions interview questions

16. What is a Closure in JavaScript?

Answer:
A closure is the combination of a function and its lexical environment. It gives access to an outer function’s scope from an inner function even after the outer function has returned.

function outer() {
  let count = 0;
  return function inner() {
    count++;
    return count;
  };
}

const counter = outer();
console.log(counter()); // 1
console.log(counter()); // 2

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17. What is Scope in JavaScript?

Answer:
Scope determines the accessibility (visibility) of variables:

• Global Scope
• Function Scope
• Block Scope (with let and const)

let a = 10; // Global Scope

function foo() {
  let b = 20; // Function Scope
  if (true) {
    let c = 30; // Block Scope
  }
}

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18. What is the difference between null and undefined?

Answer:

• undefined: A variable declared but not assigned a value.
• null: An assignment value that represents no value.

let a;
console.log(a); // undefined

let b = null;
console.log(b); // null

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19. What is an IIFE (Immediately Invoked Function Expression)?

Answer:
An IIFE is a function that runs as soon as it is defined. It avoids polluting the global scope.

(function () {
  console.log("IIFE executed");
})();

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20. What is the Prototype in JavaScript?

Answer:
All JavaScript objects inherit properties and methods from a prototype. This allows inheritance and method sharing.

function Person(name) {
  this.name = name;
}

Person.prototype.greet = function () {
  return `Hello, ${this.name}`;
};

const john = new Person("John");
console.log(john.greet()); // Hello, John

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21. Explain prototypal inheritance.

Answer:
JavaScript uses prototypal inheritance to share properties between objects. Objects inherit directly from other objects using their prototype chain.

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22. What is Currying in JavaScript?

Answer:
Currying is a function that returns another function until all arguments are provided.

function add(a) {
  return function (b) {
    return function (c) {
      return a + b + c;
    };
  };
}

console.log(add(1)(2)(3)); // 6

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23. What is memoization in JavaScript?

Answer:
Memoization is an optimization technique that caches the results of function calls.

function memoize(fn) {
  const cache = {};
  return function (n) {
    if (cache[n]) return cache[n];
    cache[n] = fn(n);
    return cache[n];
  };
}

const factorial = memoize(function(n) {
  return n <= 1 ? 1 : n * factorial(n - 1);
});

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24. What are higher-order functions?

Answer:
Functions that take other functions as arguments or return functions.

function operate(fn, a, b) {
  return fn(a, b);
}

console.log(operate((x, y) => x + y, 5, 3)); // 8

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25. Explain the concept of this in JavaScript.

Answer:
this refers to the object from which the function was called. It varies based on context:

• In global scope: this is window (in browsers)
• In method: this is the object
• In arrow functions: this is lexically bound

const obj = {
  name: 'Alice',
  greet() {
    return `Hi, I'm ${this.name}`;
  }
};

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⛓️ More Topics Coming in Part 3

Javascript functions interview questions In Part 3, we’ll cover:

• Object-Oriented JavaScript
• Functional Programming
• DOM Manipulation
• Event Handling
• Error Handling
• Built-in Methods
• Array/Object Methods

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javascript basics interview questions Part 1: Mastering the Basics

Javascript functions interview questions
Part 3, diving into OOP and functional programming in JavaScript.